Medical devices having inorganic particle layers

ABSTRACT

An expandable medical device having a particle layer disposed over a reservoir containing a therapeutic agent. The particle layer has a first porosity when the medical device is in the unexpanded configuration and a second porosity when the medical device is in the expanded configuration. The particle layer comprises a plurality of micron-sized or nano-sized particles. In certain embodiments, the particles are not connected to each other, and as such, the different porosities are provided by changes in the spacing between the particles as the medical device is expanded/unexpanded. Also disclosed are medical devices having a particle layer, wherein the particle layer comprises a plurality of encapsulated particles, and methods of coating medical devices with particles.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. provisional application Ser. No. 61/047,495 filed Apr. 24, 2008, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to medical devices, and in particular, medical devices having a coating containing a therapeutic agent.

BACKGROUND

Many implantable medical devices are coated with drugs that are eluted from the medical device upon implantation. For example, some vascular stents are coated with a drug which is eluted from the stent for treatment of the vessel and/or to prevent some of the unwanted effects and complications of implanting the stent. In such drug-eluting medical devices, various methods have been proposed to provide a mechanism for drug elution. However, there is a continuing desire for improved devices and methods for providing drug elution from medical devices.

SUMMARY

In one aspect, the present invention provides an expandable medical device having an expanded configuration and an unexpanded configuration, the medical device comprising: (a) a reservoir containing a therapeutic agent; and (b) a particle layer disposed over the reservoir, wherein the particle layer comprises a plurality of particles comprised of an inorganic material; wherein the particle layer has a first porosity when the medical device is in an unexpanded configuration and a second porosity when the medical device is in an expanded configuration, wherein the second porosity is greater than the first porosity.

In another aspect, the present invention provides a medical device comprising: a plurality of first encapsulated particles disposed over a surface of the medical device, wherein the first encapsulated particles comprise a core comprising a first therapeutic agent and a polymeric material, and wherein the core is coated with a coating comprising an inorganic material.

In another aspect, the present invention provides a method of coating a medical device, comprising: (a) providing a medical device; (b) providing a plurality of particles comprising an inorganic material; and (c) depositing the particles over a surface of the medical device.

In another aspect, the present invention provides a method of coating an expandable medical device having an unexpanded configuration and an expanded configuration, the method comprising: (a) providing a medical device having a reservoir containing a therapeutic agent; (b) with the medical device in the expanded configuration, disposing a particle layer over the reservoir, wherein the particle layer comprises a plurality of particles comprised of an inorganic material; and (c) changing the configuration of the medical device to the unexpanded configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show strut portions of a stent having a particle layer according to an embodiment of the present invention. FIG. 1A shows the strut portion with the stent in an unexpanded configuration. FIG. 1B shows the strut portion with the stent in an expanded configuration.

FIG. 2 shows the strut portion of FIGS. 1A and 1B above, but with a different distribution of the nanoparticles in the particle layer.

FIGS. 3A and 3B show strut portions of a stent having a particle layer according to another embodiment. FIG. 3A shows the strut portion with the stent in an unexpanded configuration. FIG. 3B shows the strut portion with the stent in an expanded configuration.

FIGS. 4A-4C show a method of coating a stent according to another embodiment. FIG. 4A shows a strut portion of the stent being coated with the stent in the expanded configuration. FIG. 4B shows the strut portion after the stent has been coated and with the stent returned to the unexpanded configuration. FIG. 4C shows the strut portion after the stent has been expanded again.

FIGS. 5A and 5B show strut portions of a stent having a particle layer according to another embodiment. FIG. 5A shows the strut portion with the stent in an unexpanded configuration. FIG. 5B shows the strut portion with the stent in an expanded configuration.

FIG. 6A shows a strut portion of a stent having a particle layer according to another embodiment. FIG. 6B shows a close-up cross-section view of an encapsulated particle.

FIG. 7A shows a strut portion of a stent having a particle layer according to another embodiment. FIG. 7B shows a close-up cross-section view of another encapsulated particle. FIG. 7C shows a close-up cross-section view of yet another encapsulated particle.

DETAILED DESCRIPTION

In one aspect, the present invention provides a medical device having a reservoir containing a therapeutic agent. The reservoir containing the therapeutic agent may be provided in any of various ways. The reservoir may be the therapeutic agent formulation alone, or may comprise any structure that retains or holds the therapeutic agent. For example, the reservoir may be a polymer layer or other type of layer over the medical device, with the therapeutic agent disposed therein. In another example, the reservoir may be created in the surface of the medical device (e.g., a porous surface), or the medical device may have pits, pores, cavities, or holes that contain the therapeutic agent.

The medical device is an expandable medical device and has a particle layer disposed over the reservoir, wherein the particle layer comprises a plurality of particles comprised of an inorganic material. The thickness of the particle layer will vary according to the particular application. For example, for vascular stents, the thickness of the particle layer may be in the range of 5 nm to 500 nm, but other thicknesses are also possible. In some cases, the particle layer is a monolayer of the particles.

The inorganic material may be selected on the basis of various considerations, depending upon the particular application. For example, the inorganic material may be selected for its biologic properties, structural properties, chemical properties, handling properties, or the deposition techniques that can be used. Depending upon the particular application, suitable inorganic materials for use in the barrier layer include inorganic elements, such as pure metals including chromium, gold, hafnium, iridium, niobium, palladium, platinum, tantalum, titanium, tungsten, zirconium, and alloys of these metals (e.g., nitinol); and inorganic compounds, such as metal oxides (e.g., iridium oxide or titanium oxide), metal nitrides, and metal carbides, as well as inorganic silicides.

The expandable medical device has an expanded configuration and an unexpanded configuration. The particle layer has a first porosity when the medical device is in the unexpanded configuration and a second porosity when the medical device is in the expanded configuration, with the second porosity being greater than the first porosity. As used herein, the “porosity” of the particle layer refers to the ability of a fluid, including liquids and gases, to pass through the particle layer. As such, the porosity of the particle layer will affect the release characteristics of the therapeutic agent contained in the reservoir. Various degrees of porosity are possible for the first and second porosities of the particle layer. In some cases, the first porosity does not provide a therapeutically effective release profile of the therapeutic agent (e.g., zero or negligible permeability), whereas the second porosity does provide a therapeutically effective release profile of the therapeutic agent.

The particles are micron-sized microparticles or nano-sized nanoparticles. As such, in some cases, the particles are nanoparticles have a size less than 500 nm, as measured along the smallest axis; and in some cases, the particles are nanoparticles having a size less than 50 nm, as measured along the smallest axis. The particles may have various dimensions, shapes, or geometries. For example, the particles may be spherical, columnar, rectangular, square, cylindrical, fibrous, etc. The particles may also have a monodisperse or polydisperse size distribution; and may be single particles, or formed from an aggregate of subparticles. The particles may be formed using any of various techniques for generating microparticles or nanoparticles. For example, nanoparticles may be formed using a gas-phase nucleation and growth process, which may involve evaporation-condensation, laser pyrolysis, or thermal plasma expansion.

In certain embodiments, the particles in the particle layer are not connected to each other. As such, the second porosity is provided by the enlargement of the spacing between the particles when the medical device is expanded. As used herein, particles that are “not connected” means that the particles are not adhered to each other by covalent bonds. The particles may, however, interact with each other through weaker bonding interactions, such as electrostatic attraction or hydrophilic/hydrophobic interactions.

The particles are adhered to the surface upon which the particles are deposited. The particles may be adhered to the surface via any of various types of bonding interactions, including covalent bonding, electrostatic attraction, or mechanical anchoring (e.g., the particles are at least partially embedded in the surface). The type of bonding will depend upon various factors, including the manner in which the particles are deposited onto the surface, the characteristics of the surface (e.g., composition, roughness, etc.), and the characteristics of the particles (e.g., mass, composition, size, etc.).

For example, referring to the embodiment shown in FIG. 1A, a strut portion 10 of a stent is coated with a polymer layer 12 comprising a polymeric material and a therapeutic agent. A particle layer 20 is disposed on polymer layer 12. Particle layer 20 comprises a plurality of densely packed nanoparticles 22 formed of an inorganic material. Nanoparticles 22 are adhered to polymer layer 20 with some of nanoparticles 22 being partially embedded in the surface of polymer layer 20. Nanoparticles 22 are not connected to each other, and as such, nanoparticles 22 are only loosely associated with each other even though they are densely packed. Because of the dense packing of nanoparticles 22, particle layer 20 has a relatively lower porosity to the therapeutic agent contained in polymer layer 12.

In operation, the stent is delivered to a target site. As shown in FIG. 1B, upon expansion of the stent and local deformation of strut portion 10, the spacing between nanoparticles 22 enlarge, thereby creating new pores 24 in particle layer 20 or increasing the size of already pre-existing pores, which thereby provides particle layer 20 with a relatively higher porosity to the therapeutic agent contained in polymer layer 12. This higher porosity provides a therapeutic agent release characteristic suitable for the treatment of diseased blood vessels.

Local deformation of strut portion 10 can result in various distributions of nanoparticles 22. For example, nanoparticles 22 may separate uniformly from each other or separate in clusters. In cases where nanoparticles 22 separate in clusters, the gaps created between the clusters of nanoparticles 22 can represent fractures lines in particle layer 20. In general, the distribution of the particles in the particle layer will vary depending upon various factors, such as the characteristics of the particles themselves or their adherence to the underlying substrate surface. For example, the clustering of the particles can be varied (and also predetermined) by introducing a small amount of particles of a different type from the others (e.g., different size or composition). The clustering of the particles may be random, semi-random, or uniform; and the number of particles in each cluster can vary from a single particle to many particles (e.g., in the hundreds or thousands, which can result in grain-sized clusters). For example, FIG. 2 shows an alternate distribution of nanoparticles 22 on polymer layer 12. Upon local deformation of the stent strut, the monolayer of nanoparticles 22 on polymer layer 12 may separate in groups of 4 on average (panel A, with the groups of nanoparticles encircled), in groups of 6 on average (panel B), or in groups of 8 on average (panel C), with fracture lines 26 being created between the clusters of nanoparticles 22.

In another example, referring to the embodiment shown in FIGS. 3A and 3B, a strut portion 30 of a stent is coated with a polymer layer 32 comprising a polymeric material and a therapeutic agent. A particle layer 40 is disposed on polymer layer 32. Particle layer 40 comprises a plurality of densely packed elongated nanoparticles 42 formed of an inorganic material. Nanoparticles 42 are adhered to the surface of polymer layer 32 by electrostatic attraction. Nanoparticles 42 are separated from each other by a relatively smaller space 44. Because of the dense packing of nanoparticles 42, particle layer 40 has a relatively lower porosity to the therapeutic agent contained in polymer layer 32.

In operation, the stent is delivered to a target site. As shown in FIG. 3B, upon expansion of the stent, the spacing 44 between nanoparticles 42 enlarge. Because of the enlarged spacing 44, particle layer 40 now has a relatively higher porosity to the therapeutic agent contained in polymer layer 32. This higher porosity provides a therapeutic agent release characteristic suitable for the treatment of diseased blood vessels.

The particles are deposited using any of various techniques for the deposition of microparticles or nanoparticles, including inkjet spray, electrospray, or roll coating (e.g., pressing the particles into a polymer layer on the medical device). In some cases, where the reservoir is a polymer layer on the medical device, the polymer layer may be heated to soften the polymer layer prior to depositing the particles. With the polymer layer in a softened condition, the particles may be able to better adhere to the polymer layer.

In certain embodiments, the particles are deposited by high speed impaction of the particles onto the surface being coated. The particles are accelerated and impacted onto the surface at a speed sufficient to cause the particles to adhere to the surface being coated. In some cases, the particles may be accelerated to supersonic speeds. In some cases, the particle speed may be in the range of 500 to 5,000 meters/second, but other particle speeds are also possible, depending upon the particular application. The high speed impaction causes the particles to adhere to the surface in various ways, including mechanical anchoring (e.g., embedding within the surface), microwelding, or by chemical reactions that take place in the intense conditions (e.g., extreme high temperature and density changes) created locally by the impact energy.

Any of the various known methods for accelerating particles are suitable for use in the present invention, including those using electrostatic forces, magnetic forces, and high velocity gas jets. In some cases, the particles may be ionized or electrostatically charged and then accelerated by an electric field gradient. For example, the particles may be created and deposited using the nanocluster deposition apparatus provided by Mantis Deposition Ltd. (U.K.), in which a high-pressure sputtering source is used to generate ionized nanoparticles within a gas-phase condensation zone. The ionized nanoparticles are accelerated by an electrostatic force established by creating a high voltage gradient with the surface being coated. Also suitable for use in the present invention is the method described in WO 2007/034167 (Mantis Deposition, Ltd.), in which an antibacterial layer is deposited on a substrate by generating a cloud of charged nanoparticles and electrostatically accelerating the nanoparticles toward the surface.

In some cases, the particles may be created and deposited using a hypersonic plasma particle deposition process as disclosed in U.S. Pat. No. 5,874,134 (Rao et al.), in which vapor phase reactants are injected into a thermal plasma, which is then expanded to low pressure through a nozzle. Rapid cooling in the nozzle expansion drives the nucleation of nanoparticles, which are then accelerated in a hypersonic gas jet. In some cases, the particles may be created and deposited using a focused particle beam deposition process as described in U.S. Pat. No. 6,924,004 (Rao et al.), in which gas-borne nanoparticles are generated by means of a thermal plasma expansion reactor. The particles are confined in a narrow, high-speed particle beam by passing the aerosol flow through an aerodynamic focusing stage, followed by high-speed impaction of the tightly focused particles onto a target substrate in a vacuum deposition chamber.

In certain embodiments, the particle layer is formed with the medical device in the expanded configuration. The particles are deposited in such a manner that there are a plurality of interstitial spaces between the particles. The particle layer, as now formed with the medical device in the expanded configuration, is relatively more porous to the passage of therapeutic agents from the reservoir. After the particle layer is formed, the medical device is returned to its unexpanded configuration, which causes the interstitial spaces between the particles to become smaller, thereby causing the particle layer to have a relatively lower porosity to the therapeutic agent.

For example, referring to the embodiment shown in FIGS. 4A-4C, a strut portion 70 of a stent is coated with a polymer layer 72 containing a therapeutic agent. FIG. 4A shows the stent in the expanded configuration, such that strut portion 70 has an increased curvature. With the stent in the expanded configuration, nanoparticles 74 comprising an inorganic material are deposited onto the surface of polymer layer 72 by high speed impaction. As such, nanoparticles 74 become adhered to polymer layer 72 with some of nanoparticles 74 becoming partially embedded in polymer layer 72. As a result, a particle layer 76 having interstitial spaces 78 is formed on polymer layer 72.

Referring to FIG. 4B, after particle layer 76 is formed, the stent is returned to its unexpanded configuration. In the unexpanded configuration, compressive force causes particle layer 76 to contract such that the interstitial spaces between nanoparticles 74 are substantially closed. In operation, the stent (in the unexpanded configuration) is delivered to a target site and expanded. As shown in FIG. 4C, upon expansion of the stent, particle layer 76 is also expanded such that the interstitial spaces 78 between particles 74 are opened, allowing the passage of therapeutic agent from polymer layer 72.

In certain embodiments, the particles may be deposited in such a manner as to form columnar structures on the surface. The columnar structures can be disposed on the surface in any of various orientations, and in some cases, the columnar structures are oriented substantially perpendicularly to the surface.

The columnar structures are not connected to each other. As used herein, columnar structures that are “not connected” means that the columnar structures are not adhered to each other by covalent bonds. The columnar structures may, however, interact with each other through weaker bonding interactions, such as electrostatic attraction or hydrophilic/hydrophobic interactions. As such, the second porosity is provided by the enlargement of the spacing between the columnar structures when the medical device is expanded. The particles that form the columnar structures may or may not be adherent to each other. The columnar structures can be formed from particles using any of the above-mentioned particle deposition techniques.

For example, referring to the embodiment shown in FIGS. 5A and 5B, a strut portion 50 of a stent is coated with a polymer layer 52 containing a therapeutic agent. Disposed on polymer layer 52 is a particle layer 60 comprising a plurality of nanoparticles, which are stacked upon each other to form columnar structures 62. Columnar structures 62 comprise smaller particles 64, intermediate particles 65, and larger particles 66. Smaller particles 64 are deposited onto polymer layer 52 by high speed impaction and become embedded in polymer layer 52. As a result, smaller particles 64 are adhered to polymer layer 52 with some of smaller particles 64 being embedded in polymer layer 52.

Next, intermediate particles 65 are deposited onto smaller particles 64 by high speed impaction. The high kinetic energy of the impact causes intermediate particles 65 to become microwelded to smaller particles 64. Next, larger particles 66 are deposited by acceleration and impaction, but with a kinetic energy lower than that used to deposit intermediate particles 65 and smaller particles 64. Because of the dense packing of larger particles 66, which are abutting each other, particle layer 60 has a relatively lower porosity. In operation, the stent is delivered to a target site. As shown in FIG. 5B, upon expansion of the stent, the spacing between columnar structures 62 enlarge, creating gaps 68 in particle layer 60. Gaps 68 allow the passage of therapeutic agent from polymer layer 52 through particle layer 60.

In another aspect, the present invention provides a medical device with a particle layer disposed over a surface of the medical device, wherein the particle layer comprises a plurality of encapsulated particles. The encapsulated particles have a polymer core comprised of a polymeric material and a therapeutic agent. The polymer core is encapsulated within a coating comprising an inorganic material. The medical device may or may not have a separate reservoir containing a separate source of the therapeutic agent (which may or may not be the same as the therapeutic agent contained in the encapsulated particles). The medical device may or may not be an expandable medical device.

The encapsulated particles may be formed using any of the various ways by which a therapeutic agent is made into a particulate formulation. For example, the encapsulated particles may be created by using a conventional drug particle coating process in which a particulate formulation of the therapeutic agent (with excipients such as binders, plasticizers, etc.) is coated by a fluidized bed spray coating, electroless plating, or vapor deposition technique. The encapsulated particles may have various dimensions, shapes, or geometries. For example, the encapsulated particles may be spherical, columnar, rectangular, square, cylindrical, fibrous, etc. The encapsulated particles may be micron-sized or nano-sized.

The encapsulated particles may be deposited onto the surface of the medical device in any of various ways, including those described above. In cases where the particles are deposited using an electrospray process, the target surface of the medical device may be positioned at a distance far enough away that the particles are dry when hitting the target. In some cases, the encapsulated particles are not connected to each other.

The therapeutic agent is released from the encapsulated particles in any of various ways. In some cases, the coating of inorganic material may be porous, allowing the passage of the therapeutic agent from the polymer core. In some cases, where the medical device is implantable, the coating is designed to break under the pressures applied during the implantation. For example, for a vascular stent, the implantation pressure during balloon-deployment of the stent may cause the coating to crack open. In some cases, different encapsulated particles may be used to contain different therapeutic agents; and further, the different therapeutic agents may be released according to different release profiles.

For example, referring to the embodiment shown in FIGS. 6A and 6B, a strut portion 80 of a stent is coated with a polymer layer 82 containing a therapeutic agent. Disposed on polymer layer 82 is a particle layer 84 formed from a plurality of micron-sized encapsulated particles 86. Encapsulated particles 86 are deposited onto polymer layer 82 by high speed impaction. As a result, encapsulated particles 86 are adhered to polymer layer 82 with some of encapsulated particles 86 being partially embedded in the surface of polymer layer 82. Referring to FIG. 6B, encapsulated particles 86 have a core 88 formed of a polymeric material and a therapeutic agent. Core 88 is encapsulated within a porous coating 87 of inorganic material. In operation, the stent is delivered to a target site. Upon implantation of the stent, the therapeutic agent contained in core 88 passes through porous coating 87 and is delivered to the surrounding tissue.

In another example, referring to the embodiment shown in FIGS. 7A-7C, a particle layer 92 is disposed on a strut portion 90 of a stent. Particle layer 92 is formed from a plurality of first encapsulated particles 94, which are spherically-shaped, and second encapsulated particles 95, which are rectangularly-shaped. These encapsulated particles are micron-sized and deposited onto the bare surface of strut portion 90 by high speed impaction. As a result, the particles are adhered to the bare surface of strut portion 90 with some of the particles being partially embedded in the surface.

Referring to FIG. 7B, first encapsulated particles 94 have a spherically-shaped core 97 formed of a polymeric material and a first therapeutic agent. Core 97 is encapsulated within a porous coating 96 of an inorganic material. Second encapsulated particles 95 have a rectangularly-shaped core 98 formed of a polymeric material and a second therapeutic agent, which is different from the first therapeutic agent. Core 98 is surrounded by an impermeable, but brittle coating 99 formed of an inorganic material. In operation, the stent is delivered to a target site. During balloon expansion of the stent, strut portion 90 is pressed against the wall of a blood vessel. This pressure causes coating 99 in the second encapsulated particles 95 to crack, providing an immediate release of the second therapeutic agent. Over a slower time frame, the first therapeutic agent contained in core 98 of first encapsulated particles 94 is released as the first therapeutic agent passes through porous coating 96. In an alternative embodiment, the coating 99 in second encapsulated particles 95 may be porous and have a porosity that is different from coating 96 in first encapsulated particles 94. As a result, the second therapeutic agent has a different release profile than the first therapeutic agent.

Non-limiting examples of medical devices that can be used with the present invention include stents, stent grafts, catheters, guide wires, neurovascular aneurysm coils, balloons, filters (e.g., vena cava filters), vascular grafts, intraluminal paving systems, pacemakers, electrodes, leads, defibrillators, joint and bone implants, spinal implants, access ports, intra-aortic balloon pumps, heart valves, sutures, artificial hearts, neurological stimulators, cochlear implants, retinal implants, and other devices that can be used in connection with therapeutic coatings. Such medical devices are implanted or otherwise used in body structures, cavities, or lumens such as the vasculature, gastrointestinal tract, abdomen, peritoneum, airways, esophagus, trachea, colon, rectum, biliary tract, urinary tract, prostate, brain, spine, lung, liver, heart, skeletal muscle, kidney, bladder, intestines, stomach, pancreas, ovary, uterus, cartilage, eye, bone, joints, and the like.

The therapeutic agent used in the present invention may be any pharmaceutically acceptable agent (such as a drug), a biomolecule, a small molecule, or cells. Exemplary biomolecules include peptides, polypeptides and proteins; antibodies; oligonucleotides; nucleic acids such as double or single stranded DNA (including naked and cDNA), RNA, antisense nucleic acids such as antisense DNA and RNA, small interfering RNA (siRNA), and ribozymes; genes; carbohydrates; angiogenic factors including growth factors; cell cycle inhibitors; and anti-restenosis agents. Exemplary small molecules include hormones, nucleotides, amino acids, sugars, and lipids and compounds have a molecular weight of less than 100 kD. Exemplary cells include stem cells, progenitor cells, endothelial cells, adult cardiomyocytes, and smooth muscle cells.

A reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one of the element is present. Rather, the article “a” or “an” is intended to mean one or more (or at least one) unless the text expressly indicates otherwise. The terms “first,” “second,” and so on, when referring to an element, are not intended to suggest a location or ordering of the elements. Rather, the terms are used as labels to facilitate discussion and distinguish elements from one another.

The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Each of the disclosed aspects and embodiments of the present invention may be considered individually or in combination with other aspects, embodiments, and variations of the invention. Modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art and such modifications are within the scope of the present invention. 

1. An expandable medical device having an expanded configuration and an unexpanded configuration, the medical device comprising: a reservoir containing a therapeutic agent; and a particle layer disposed over the reservoir, wherein the particle layer comprises a plurality of particles comprised of an inorganic material; wherein the particle layer has a first porosity when the medical device is in an unexpanded configuration and a second porosity when the medical device is in an expanded configuration, wherein the second porosity is greater than the first porosity.
 2. The medical device of claim 1, wherein the inorganic material is a metal or metal oxide.
 3. The medical device of claim 1, wherein the first porosity is negligible or zero.
 4. The medical device of claim 1, wherein the particles are not connected to each other, and wherein the spacing between the particles enlarge when the medical device is expanded.
 5. The medical device of claim 1, wherein the reservoir is a polymer layer comprising a polymeric material and the therapeutic agent.
 6. The medical device of claim 5, wherein the particles are adhered to the polymer layer.
 7. The medical device of claim 6, wherein at least some of the particles are embedded in the polymer layer.
 8. The medical device of claim 1, wherein the particles form columnar structures.
 9. The medical device of claim 8, wherein the columnar structures are not connected to each other, and wherein the spacing between the columnar structures enlarge when the medical device is expanded.
 10. The medical device of claim 1, wherein the particles are nanoparticles.
 11. A medical device comprising: a plurality of first encapsulated particles disposed over a surface of the medical device, wherein the first encapsulated particles comprise a core comprising a first therapeutic agent and a polymeric material, and wherein the core is coated with a coating comprising an inorganic material.
 12. The medical device of claim 11, wherein the encapsulated particles are not connected to each other.
 13. The medical device of claim 11, further comprising a plurality of second encapsulated particles disposed over the surface of the medical device, wherein the second encapsulated particles comprise a core comprising a second therapeutic agent and a polymeric material, and wherein the core is coated with a coating comprising an inorganic material.
 14. The medical device of claim 13, wherein the porosity of the coating of the first encapsulated particles is different from the porosity of the coating of the second encapsulated particles.
 15. The medical device of claim 13, wherein the first therapeutic agent is different from the second therapeutic agent.
 16. The medical device of claim 11, wherein the coating is porous.
 17. The medical device of claim 11, wherein the medical device is an implantable medical device, and wherein the coating breaks under the pressure applied during implantation.
 18. The medical device of claim 11, wherein the encapsulated particles are adhered to the bare surface of the medical device.
 19. The medical device of claim 11, wherein the encapsulated particles are adhered to the surface of a polymer layer on the medical device, and wherein the polymer layer comprises a polymeric material and an additional therapeutic agent.
 20. The medical device of claim 11, wherein the inorganic material is a metal or metal oxide.
 21. A method of coating a medical device, comprising: providing a medical device; providing a plurality of particles comprising an inorganic material; and depositing the particles over a surface of the medical device.
 22. The method of claim 21, wherein the step of depositing the particles comprises: accelerating the particles towards the surface of the medical device to be coated; and impacting the particles onto the surface to be coated.
 23. The method of claim 22, wherein the particles are accelerated to a speed that is sufficient to cause the particles to adhere to the surface upon impaction.
 24. The method of claim 23, wherein the particles are accelerated to a supersonic speed.
 25. The method of claim 22, wherein the step of accelerating the particles comprises ionizing or electrostatically charging the particles.
 26. The method of claim 25, further comprising applying a bias voltage to the medical device.
 27. The method of claim 22, wherein the step of accelerating the particles comprises entraining the particles in a high velocity gas jet.
 28. The method of claim 21, wherein the step of providing the particles comprises forming the particles.
 29. The method of claim 28, wherein the particles are nanoparticles that are formed using a gas-phase nucleation and growth process.
 30. The method of claim 21, wherein the medical device has a polymer layer comprising a polymeric material and the therapeutic agent, and wherein the surface to be coated is a surface of the polymer layer.
 31. The method of claim 30, further comprising heating the polymer layer to soften the polymer layer prior to depositing the particles.
 32. The method of claim 30, wherein the particles are deposited by roll-coating.
 33. The method of claim 30, wherein the particles are deposited by electrostatic spray.
 34. The method of claim 30, wherein the particles are deposited by inkjet spray.
 35. The method of claim 21, wherein the particles are encapsulated particles comprising a core comprising a therapeutic agent and a polymeric material, and wherein the core is coated with a coating comprising the inorganic material.
 36. The method of claim 21, wherein the inorganic material is a metal or metal oxide.
 37. A method of coating an expandable medical device having an unexpanded configuration and an expanded configuration, the method comprising: providing a medical device having a reservoir containing a therapeutic agent; with the medical device in the expanded configuration, disposing a particle layer over the reservoir, wherein the particle layer comprises a plurality of particles comprised of an inorganic material; and changing the configuration of the medical device to the unexpanded configuration.
 38. The method of claim 37, wherein the step of disposing the particle layer comprises depositing the particles over the reservoir.
 39. The method of claim 37, wherein the reservoir is a polymer layer comprising a polymeric material and the therapeutic agent.
 40. The method of claim 39, wherein the particles are adhered to the polymer layer.
 41. The method of claim 40, wherein the step of disposing the particle layer comprises depositing the particles onto the polymer layer, and further comprising heating the polymer layer to soften the polymer layer prior to depositing the particles.
 42. The method of claim 37, wherein the particle layer has a first porosity when the medical device is in an unexpanded configuration and a second porosity when the medical device is in an expanded configuration, wherein the second porosity is greater than the first porosity.
 43. The method of claim 37, wherein the particle layer, as disposed over the medical device in the expanded configuration, has a plurality of interstitial spaces between the particles.
 44. The method of claim 43, wherein the interstitial spaces become smaller when the medical device is changed to the unexpanded configuration.
 45. The method of claim 37, wherein the particles are deposited by high speed impaction.
 46. The method of claim 37, wherein the particles are deposited by roll-coating.
 47. The method of claim 37, wherein the particles are deposited by electrostatic spray.
 48. The method of claim 37, wherein the particles are deposited by inkjet spray.
 49. The method of claim 37, wherein the inorganic material is a metal or metal oxide.
 50. The method of claim 37, wherein the particles are not connected to each other. 